ITS CHARACTERISTIC ANIMALS. 
205 
nearly completed, leaving only a narrow gulf 
running up to the neighborhood of St. Louis to 
be filled by modern detritus, and the peninsula 
of Florida to be built by the industrious Coral- 
Workers of our own period. The age of the Al- 
leghany chain is not yet positively determined, 
but it was probably raised at the close of the Car- 
boniferous epoch. Up to that time, only the 
Laurentian Hills, the northern side of that moun- 
tainous triangle which now makes the skeleton, 
as it were, of the United States, existed. The 
upheaval of the Alleghanies added its eastern 
side, raising the central part of the continent so 
as to form a long slope from the base of the Al- 
leghanies to the Pacific Ocean ; but it was not 
until the Tertiary Age that the upheaval of the 
great chain at the West completed the triangle, 
and transformed that wide westerly slope into the 
Mississippi Valley, bounded on one side by the 
Alleghanies, and on the other by the Rocky 
Mountains. 
It is my belief, founded upon the tropical char- 
acter of the Fauna, that a much milder climate 
then prevailed over the whole northern hemis- 
phere than is now known to it. Some natural- 
ists have supposed that the presence of the tro- 
pical Mammalia in the Northern Temperate Zone 
might be otherwise accounted for, — that they 
might have been endowed with warmer covering, 
